1. Field of the Invention
The present invention generally relates to an optical multiplexing/demultiplexing device with a variable branching ratio or an optical coupler with a variable coupling ratio that is used with an optical fiber communication system. More particularly, the present invention relates to an optical multiplexing/demultiplexing device with a variable branching ratio that is capable of continuously changing a distribution ratio F2I:F3I of light intensities of two optical fiber circuits F2 and F3 from 0:100 to 100:0 so as to provide branched outputs when distributing light from a single optical fiber F1 to other two optical fiber circuits F2 and F3, or a similar optical coupler with variable coupling ratio.
2. Description of the Related Art
A conventional optical multiplexing/demultiplexing device with a variable branching ratio will be described. A multiplexing/demultiplexing device with a variable branching ratio shown in FIG. 9 through FIG. 11 is the optical multiplexing/demultiplexing device with a variable branching ratio in accordance with a prior invention (U.S. Pat. No. 5,050,950) by the inventors of the present application. FIG. 9 is a side sectional view of the multiplexing/demultiplexing device with a variable branching ratio, FIG. 10 is a sectional view taken at the line AA in a central portion at the same branching ratio, and FIG. 11 is a sectional view taken at the line AA in a central portion at a different branching ratio. The multiplexing/demultiplexing device with a variable branching ratio with an abraded block is constructed by a block B1 with an optical fiber F1 and a block B2 with an optical fiber F2 that are opposed to each other and brought in contact. A clad of the optical fiber F1 has an optical fiber core 7 at its center, and a clad of the optical fiber F2 has an optical fiber core 8 at its center. Block members 1 and 2 are provided with V-grooves 3 and 4, respectively, and the optical fibers F1 and F2 are fitted in the V-grooves with their bellies curved as illustrated and adhesively secured. The block B1 with the optical fiber is abraded as follows. First, one end of the optical fiber F1 of the block member 1 is coupled to a light source, while the other end is coupled to a power meter to measure loss. Under this measurement condition, a surface 5 of the block member 1 is scrubbed together with the optical fiber F1 until the loss reaches 3 dB (=50%). A central portion of the clad of the optical fiber F1 is plane-abraded to a point in the vicinity of the core 7. The block B2 with the optical fiber is also abraded in the same manner.
Next, the blocks B1 and B2 with the optical fibers are brought in close contact as shown in FIG. 10 so that they are vertically symmetrical. The optical fibers F1 and F2 are coupled by the evanescent effect, and an optical multiplexing/demultiplexing device having a branching ratio of 50:50 is formed. Fifty per cent of input light from one end of the optical fiber F1 is output to the other end of the optical fiber F1, while the other 50% of the input light is output to the optical fiber F2.
When the block B2 with the optical fiber is moved in a direction of an arrow as shown in FIG. 11, optical coupling due to the evanescent effect between the optical fiber F1 and F2 weakens in proportion to the moving distance. A quantity of light, 50%, transmitted from the optical fiber F1 to the branched side of the optical fiber F1 remains unchanged, while a quantity of light transmitted to the optical fiber F2 decreases, 50% being an upper limited. Most conventional abraded-block type optical multiplexing/demultiplexing devices with variable branching ratios are based on the variable branching ratio principle described above.
The conventional abraded-block type optical, multiplexing/demultiplexing device with a variable branching ratio set forth above poses the following problems, many of which have been verified by prototype experiments performed by the assignee:
(1) In the optical fiber multiplexing/demultiplexing device with a variable branching ratio, an optical fiber formed of fine, fragile quartz glass is curved and adhesively secured into a V-groove of a block member to polish a surface of the optical fiber. Hence, the optical fiber is frequently damaged during the process, making it difficult to achieve stable manufacture.
(2) As the optical branching ratio increases, positioning adjustment between the blocks B1 and B2 with optical fibers requires higher accuracy in units of 0.1 xcexcm, making it extremely difficult to perform adjustment. In addition to the need for highly accurate positioning, variations in the optical branching ratio characteristics caused by an ambient temperature and external forces increase, posing disadvantages from a viewpoint of reliability.
(3) From an aspect of operating principle, when the optical branching ratio is 50:50, there should not be a great optical insertion loss. For example, when an optical branching ratio of the branched end of the optical fiber F1 to F2 is set to 100:10, the maximum branched output 50% to the branched end of the optical fiber F1 corresponds to the optical branching ratio 100; therefore, the optical branching ratio 10 to the optical fiber F2 will be about 5%, resulting in a 45% optical insertion loss with respect to the original quantity of light of the optical fiber F1 at the input end.
Accordingly, it is an object of the present invention to provide an optical multiplexing/demultiplexing device with a variable branching ratio based on a new principle that has overcome the difficulties in the conventional optical multiplexing/demultiplexing device with a variable branching ratio (abraded-block type) described above.
To this end, according to one aspect of the present invention, there is provided an optical multiplexing/demultiplexing device with a variable branching ratio that employs a new reflection mirror system to permit a simple structure and easy mass production of its component parts.
According to another aspect of the present invention, there is provided a reflection mirror type optical multiplexing/demultiplexing device with a variable branching ratio that permits easy adjustment for setting to a high branching ratio and exhibits high reliability against environmental conditions.
According to yet another object of the present invention, there is provided a reflection mirror type optical multiplexing/demultiplexing device with a variable branching ratio that exhibits a small optical insertion loss even at a high branching ratio.
To this end, according to one aspect of the present invention, there is provided an optical multiplexing/demultiplexing device with variable branching ratio comprising a first collimator lens assembly including a first collimator lens, and first and second optical fibers F1 and F2 which have distal ends thereof optically coupled to one end face of the lens being away from each other by a distance xe2x80x9cd,xe2x80x9d an optical axis, of the lens positioned at a midpoint therebetween, a second collimator lens assembly including a second collimator lens and a third optical fiber F3 which has a distal end thereof optically coupled to one end face of the lens, being away from an optical axis of the lens by a distance xe2x80x9cd/2,xe2x80x9d aligning means for disposing the first and second collimator lens assemblies so that they oppose each other symmetrically with respect to an optical reference plane, with optical axis thereof being aligned, and for fixedly supporting the collimator lens assemblies so that an image at the distal end of the first optical fiber is formed at the distal end of the third optical fiber, reflection mirror means which is supported by the aligning means so that the reflection mirror means may move within the reference plane, and reflects and connects a part or all of a parallel beam of an expanded mode field area that is emitted from the first optical fiber and formed on the reference plane to the second optical fiber F2, or transmits and connects all or a part of the parallel beam to the third optical fiber F3, and driving means for moving the reflection mirror.
The first and second collimator lenses are rod lenses having a reference length of 0.25 pitch, and the first and second optical fibers are supported by a first ferrule, while the third optical fiber is supported b a second ferrule, these three optical fibers being coupled to the rod lenses constituting the first and second collimator lens assemblies. A shaft of the reflection mirror of the reflection mirror means is inserted in a shaft hole which is provided in the aligning means and parallel to the optical axis of the collimator lenses, and the reflection mirror means is guided by one or more guiding surfaces positioned virtually at right angles to the shaft hole of the aligning means. The shaft of the reflection mirror of the reflection mirror means has a behavior of rotating in one direction with respect to the aligning means, and the driving means for moving the reflection mirror moves the mirror means against the behavior of rotating in one direction.
The reflection mirror uses a metal, such as stainless steel, as a material thereof, and both surfaces of the metal are provided with Tixe2x80x94N coating of a hardness of MHv 1800 or more and coated with a film having high reflectivity, such as gold (Au) or platinum Pt), by sputtering or chemical plating.
Adopting the same construction as that of the, above demultiplexing device makes it possible to implement an optical coupler with a variable coupling ratio.
To this end, according to one aspect of the present invention, there is provided an optical coupler with variable coupling ratio comprising a first collimator lens assembly including a first collimator lens and a first optical fiber which has a distal end thereof optically coupled to one end face of the lens, being away from an optical axis of the lens by a predetermined distance, a second collimator lens assembly including a second collimator lens and a second optical fiber which has a distal end thereof optically coupled to one end face of the lens, being away from an optical axis of the lens by a predetermined distance, aligning means for disposing the first and second collimator lens assemblies so that they oppose each other symmetrically with respect to an optical reference plane, with optical axis thereof being aligned, and for fixedly supporting the collimator lens assemblies so that an image at the distal end of the first optical fiber is formed at the distal end of the second optical fiber, shutter means which is supported by the aligning means so that the shutter means may move within the reference plane, and reflects and connects a part or all of a parallel beam of an expanded mode field area that is emitted from the first optical fiber and formed on the reference plane to the second optical fiber, and driving means for moving the shutter.
According to another aspect of the present invention, there is provided an optical coupler with variable coupling ratio comprising a collimator lens assembly including a collimator lens and first and second optical fibers F1 and F2 which have distal ends thereof optically coupled to one end face of the lens, being away from each other by a predetermined distance with an optical axis of the lens positioned at a midpoint therebetween, aligning means for fixedly supporting the collimator lens assembly so that an optical axis of the collimator lens assembly is positioned at right angles to an optical reference plane, reflection mirror means including a reflection mirror which is supported by the aligning means so that the reflection mirror may move within the reference plane, and reflects and connects a part or all of a parallel beam of an expanded mode field area that is emitted from the first optical fiber and formed on the reference plane to the second optical fiber F2, and driving means for moving the reflection mirror.